# std::is_reference

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< cpp‎ | types

Defined in header `<type_traits>` |  |
template< class T > struct is_reference; |  | (since C++11)

`std::is_reference` is a UnaryTypeTrait.

If `T` is a reference type (lvalue reference or rvalue reference), provides the member constant `value` equal true. For any other type, `value` is false.

If the program adds specializations for `std::is_reference` or `std::is_reference_v`, the behavior is undefined.

### Template parameters

T | - | a type to check

### Helper variable template

template< class T > constexpr bool is_reference_v = is_reference<T>::value; |  | (since C++17)

## Inherited from std::integral_constant

### Member constants

value [static] | true if `T` is a reference type, false otherwise (public static member constant)

### Member functions

operator bool | converts the object to bool, returns value (public member function)
operator() (C++14) | returns value (public member function)

### Member types

Type | Definition
`value_type` | bool
`type` | std::integral_constant<bool, value>

### Possible implementation

    template<class T> struct is_reference : std::false_type {};
    template<class T> struct is_reference<T&> : std::true_type {};
    template<class T> struct is_reference<T&&> : std::true_type {};

### Example

Run this code

    #include <iostream>
    #include <type_traits>

    class A {};

    int main()
    {
    #   define REF(x) << #x " ?: " << x << '\n'
        std::cout << std::boolalpha
        REF(std::is_reference_v<A>)
        REF(std::is_reference_v<A&>)
        REF(std::is_reference_v<A&&>)
        REF(std::is_reference_v<long>)
        REF(std::is_reference_v<long&>)
        REF(std::is_reference_v<long&&>)
        REF(std::is_reference_v<double*>)
        REF(std::is_reference_v<double*&>)
        REF(std::is_reference_v<double*&&>);
    #   undef REF
    }

Output:

    std::is_reference_v<A> ?: false
    std::is_reference_v<A&> ?: true
    std::is_reference_v<A&&> ?: true
    std::is_reference_v<long> ?: false
    std::is_reference_v<long&> ?: true
    std::is_reference_v<long&&> ?: true
    std::is_reference_v<double*> ?: false
    std::is_reference_v<double*&> ?: true
    std::is_reference_v<double*&&> ?: true

### See also

is_lvalue_reference (C++11) | checks if a type is an lvalue reference (class template)
is_rvalue_reference (C++11) | checks if a type is an rvalue reference (class template)

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